1. Field of the Invention
The present invention relates to a recordable optical recording medium and, more particularly, to an optical recording medium having a light-absorbing and a light-reflecting layer formed in superposition on a light-transparent substrate.
2. Description of the Related Art
The use of organic dyes, such as cyanine and phthalocyanine dyes, in the recording membrane of recordable optical recording medium is generally known. In order to write information on such optical recording media, a laser beam is focused in a small area of the recording membrane and converted to thermal energy which changes the characteristics of the recording membrane or layer (i.e., forming a pit). The common practice adopted to insure a smooth change in the properties of the recording membrane is to prepare two substrates each bearing a recording membrane and these substrates are combined together with the two recording membranes facing each other, thereby providing a so-called "air sandwich" structure.
The laser beam used to write data on this type of optical recording media can be aimed into the outer face of either transparent substrate so as to form an optically readable pit in either of the recording membranes. In order to reproduce the recorded data, a reading laser beam having a lower power than the writing beam is focused on the pit surface and the contrast between the area where the pit is formed and the area where no pits are formed is read as an electric signal.
Media of the read-only memory (ROM) type having prerecorded data are also available and have been commercially used in audio recording and information processing fields. The ROM media have no recording membrane into which data can be recorded. Stated more specifically, prepits which correspond to the data to be reproduced are already formed on the plastic substrate by press forming with a master. Then, a reflecting layer made of a metal such as Au, Ag, Cu or Al is formed over the prepits, and then a protective layer can be formed on the reflecting layer.
A typical class of ROM media are compact disks which are commonly referred to as CDs. Information is written into and read from CDs with signals of standardized specifications. In accordance with such specifications, CD reproducing apparatus are widely used as compact disk players (CD players).
The recordable optical recording media are the same as CDs insofar as a laser beam is used for reading/writing and that both media are in a disk form. It is strongly desired to develop a recordable medium that complies with the ROM or CD specifications and, as a result, is adapted for use in CD players. However, a problem has been encountered in attempts to achieve this object.
In particular, if the air sandwich structure, which is widely adopted in the conventional recordable media, is merely replaced by the provision of a light-reflecting layer on the conventional recording membrane, the reflectance and modulation factor of a laser beam cannot be made sufficiently high to satisfy the CD specifications.